once.h revision d0332953cda33fb4f8e24ebff9c49159b69c43d6 
1// Protocol Buffers - Google's data interchange format
2// Copyright 2008 Google Inc.  All rights reserved.
3// http://code.google.com/p/protobuf/
4//
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6// modification, are permitted provided that the following conditions are
7// met:
8//
9//     * Redistributions of source code must retain the above copyright
10// notice, this list of conditions and the following disclaimer.
11//     * Redistributions in binary form must reproduce the above
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13// in the documentation and/or other materials provided with the
14// distribution.
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17// this software without specific prior written permission.
18//
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29// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
30
31// Author: kenton@google.com (Kenton Varda)
32//
33// emulates google3/base/once.h
34//
35// This header is intended to be included only by internal .cc files and
36// generated .pb.cc files.  Users should not use this directly.
37//
38// This is basically a portable version of pthread_once().
39//
40// This header declares three things:
41// * A type called ProtobufOnceType.
42// * A macro GOOGLE_PROTOBUF_DECLARE_ONCE() which declares a variable of type
43//   ProtobufOnceType.  This is the only legal way to declare such a variable.
44//   The macro may only be used at the global scope (you cannot create local
45//   or class member variables of this type).
46// * A function GogoleOnceInit(ProtobufOnceType* once, void (*init_func)()).
47//   This function, when invoked multiple times given the same ProtobufOnceType
48//   object, will invoke init_func on the first call only, and will make sure
49//   none of the calls return before that first call to init_func has finished.
50//
51// This implements a way to perform lazy initialization.  It's more efficient
52// than using mutexes as no lock is needed if initialization has already
53// happened.
54//
55// Example usage:
56//   void Init();
57//   GOOGLE_PROTOBUF_DECLARE_ONCE(once_init);
58//
59//   // Calls Init() exactly once.
60//   void InitOnce() {
61//     GoogleOnceInit(&once_init, &Init);
62//   }
63//
64// Note that if GoogleOnceInit() is called before main() has begun, it must
65// only be called by the thread that will eventually call main() -- that is,
66// the thread that performs dynamic initialization.  In general this is a safe
67// assumption since people don't usually construct threads before main() starts,
68// but it is technically not guaranteed.  Unfortunately, Win32 provides no way
69// whatsoever to statically-initialize its synchronization primitives, so our
70// only choice is to assume that dynamic initialization is single-threaded.
71
72#ifndef GOOGLE_PROTOBUF_STUBS_ONCE_H__
73#define GOOGLE_PROTOBUF_STUBS_ONCE_H__
74
75#include <google/protobuf/stubs/common.h>
76
77#ifndef _WIN32
78#include <pthread.h>
79#endif
80
81namespace google {
82namespace protobuf {
83
84#ifdef _WIN32
85
86struct ProtobufOnceInternal;
87
88struct LIBPROTOBUF_EXPORT ProtobufOnceType {
89  ProtobufOnceType();
90  ~ProtobufOnceType();
91  void Init(void (*init_func)());
92
93  volatile bool initialized_;
94  ProtobufOnceInternal* internal_;
95};
96
97#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME)                    \
98  ::google::protobuf::ProtobufOnceType NAME
99
100inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
101  // Note:  Double-checked locking is safe on x86.
102  if (!once->initialized_) {
103    once->Init(init_func);
104  }
105}
106
107#else
108
109typedef pthread_once_t ProtobufOnceType;
110
111#define GOOGLE_PROTOBUF_DECLARE_ONCE(NAME)                    \
112  pthread_once_t NAME = PTHREAD_ONCE_INIT
113
114inline void GoogleOnceInit(ProtobufOnceType* once, void (*init_func)()) {
115  pthread_once(once, init_func);
116}
117
118#endif
119
120}  // namespace protobuf
121}  // namespace google
122
123#endif  // GOOGLE_PROTOBUF_STUBS_ONCE_H__
124